Semiconductor transistors, in particular field-effect controlled switching devices such as a MISFET (Metal Insulator Semiconductor Field Effect Transistor), in the following also referred to as MOSFET (Metal Oxide Semiconductor Field Effect Transistor) and a HEMT (high-electron-mobility Field Effect Transistor) also known as heterostructure FET (HFET) and modulation-doped FET (MODFET) are used in a variety of applications. HEMTs are preferred in many applications due to their favorable power density, on-state resistance, switching frequency, and efficiency benefits over over conventional silicon based transistors.
HEMTs are viewed as an attractive candidate for power switching applications. A power transistor is a device that is capable of switching substantial voltages and/or currents associated with high power applications. For example, a power transistor may be required to block a voltage of at least 200 V, 400 V, 600 V or more. In addition, a power transistor may be required to conduct currents in the range of ones, tens or hundreds of amperes during normal operation. Due to the high electron mobility of the two-dimensional carrier gas in the heterojunction configuration, HEMTs offer high conduction and low losses in comparison to many conventional semiconductor transistor designs and therefore are well suited for these large operating currents.
In general, III-V semiconductor materials, such as GaN, are used to form high electron mobility semiconductor devices. With GaN technology, the presence of polarization charges and strain effects result in the realization of a two-dimensional charge carrier gas, which is a two-dimensional electron or hole inversion layer characterized by very high carrier density and carrier mobility. A two-dimensional charge carrier gas such as a 2DEG (two-dimensional electron gas) or 2DHG (two-dimensional hole gas) forms the channel region of the device. A thin, e.g. 1-2 nm, AlN layer can be provided between the GaN buffer layer and the alloy barrier layer to minimize alloy scattering and enhance 2DEG mobility.
HEMTs are typically configured as lateral devices, i.e., transistor devices in which the channel flows in a direction that is parallel to the main surface of the semiconductor substrate. Thus, HEMTs typically include electrically conductive terminals (e.g., gate, source and drain terminals) disposed on a single main surface of the semiconductor substrate that are electrically coupled to different points of the laterally extending channel. The rear side of a lateral transistor device, on the other hand, typically does not include any conductive terminals and is thus electrically inactive.
While HEMTs offer advantageous characteristics with respect to voltage and current switching capability, these devices have significantly increased power density in comparison to silicon-based counterparts. Thus, a substantial amount of heat is generated during operation of these devices, which requires innovative cooling solutions. As the technology progresses towards higher voltages and frequencies, challenges with respect to cooling become even more pronounced. Thus, package designers are seeking ways to improve cooling for GaN based HEMTs.